1. Field of the Invention
The present invention relates to a shuttle mechanism for bidirectionally moving a printing unit, and more particularly to a shuttle mechanism using a linear motor.
2. Description of the Related Art
There has been known a printer with dot print hammers for printing dot matrices on a printable medium. The dot matrices appear as characters, symbols, and the like on the printable medium. One such printer includes a shuttle mechanism using a linear motor. The shuttle mechanism drives a printing unit reciprocally and bidirectionally in a main scanning direction while the dot print hammers mounted on the printing unit are actuated.
The basic principles by which linear motors operate will be described while referring to FIGS. 1A and 1B. As shown in FIGS. 1A and 1B, a linear motor includes parallel upper and lower magnet banks 110a and 110b disposed in opposition with each other and separated by a minute space. Each of magnet banks 110a and 110b includes magnets 110' juxtaposed with alternate polarity, that is, if the inner face of one magnet 110' in a magnet bank 110a or 110b constitutes a south pole, then the inner face of the adjacent magnet in that bank constitutes a north pole, and so on. Each magnet 110' of each magnet bank, for example, in magnet bank 110a, has a corresponding magnet in the other magnet bank, for example, in magnet bank 110b. Corresponding magnets 110' are located directly opposite each other, and their facing pole faces are of opposite polarity, i.e., so that the south pole of one magnet 110' faces directly opposite the north pole of its corresponding magnet 110', and so on all along the magnet banks 110a and 110b.
A coil member 20 is disposed in the minute space between the upper magnet bank 110a and the lower magnet bank 110b. The coil member 20 has a plurality of conductors aligned parallel with alignment of the magnets 110'. Conductors of the coil member 20 are applied with current that flows perpendicular to the direction of the magnetic lines of force of the magnets in the magnet banks 110a and 110b. However, each conductor is applied with a current flowing in the opposite direction of current applied to adjacent conductors.
With the configuration shown in FIG. 1A, thrust is generated and the coil member 20 moves in the direction of force indicated by the arrow 81 in accordance with Fleming's left-hand rule. The direction of force will reverse when the coil member 20 moves to the position indicated in FIG. 1B. The coil member 20 will be reciprocally driven between the position shown in FIG. 1A and the position shown in FIG. 1B.
The strength of thrust F is represented by the following equation: EQU F=ntBLI
wherein n is the number of effective conductors mounted on the coil member 20, t is the number of turns in each conductor, B is the magnetic flux density, L is the effective length of the conductor, and I is the current.
FIG. 2 shows configuration of a conventional linear motor driven shuttle mechanism that works based on the above-described theory. Two side plates 62 and 63 are provided for supporting upper and lower yokes 30a and 30b and a guide shaft 70 aligned parallel with the alignment of the magnets 110'. The upper and lower magnet banks 110a and 110b are fixed to the upper and lower yokes 30a and 30b, respectively, in confronting relation with each other with a space therebetween. Bushes 60 attached to a base plate 50 are slidably movable along the guide shaft 70. The base plate 50 is provided for integrally connecting the coil member 20 to the bushes 60 so that the coil member 20 is suspended between the upper and lower magnet banks 110a and 110b. This configuration allows the coil member 20 to reciprocally and linearly move in parallel with alignment of the magnets 110'. Although not shown in the drawings, a printing unit such as a dot print hammer bank is secured to the base plate 50.
In order to increase magnetic flux density of the above-described mechanism, the gaps between the upper and lower surfaces of the coil member 20 and respective magnet banks 110a and 110b and between the yokes 30a and 30b are formed as narrow as possible. For example, the gaps between the coil member 20 and magnet banks 110a and 110b are usually formed to about 0.7 mm. Such narrow gaps limit ventilation so that heat builds up around the coil member 20. Even provision of a blower or cooling fan could not provide air flow sufficient to effectively cool this area.
The size of the coil 20 restricts the level of improvement in the magnetic flux density obtainable by narrowing the gaps between the two magnet banks 110a and 110b and between the two yokes 30. The magnetic flux density could be improved by changing the thickness of the magnet banks 110a and 110b and/or the material used to make the magnet banks 110a and 110b. However, such changes could be costly.